======OPAC development======
I'm writing this as I deconstruct how the current OPAC (as of January 2008) works, largely for my own memory, but perhaps this will be helpful to others.

=====How the OPAC makes OpenSRF calls=====
I needed to figure out which OpenSRF call was being made by a function in the OPAC so that I could adjust its behavior. The problem was that an OpenSRF call was returning a string literal of '0' for a "username not found" result, which in JavaScript's type coercion environment could easily be interpreted as a numeric 0, which would map to another user's ID and cause further confusion. We wanted to change the call to return an explicit ''null'' instead. Here's how I figured out what needed to change:

  - The problem was occurring on the "My OPAC" page, with a URL of ''https://localhost/opac/en-US/skin/default/xml/myopac.xml''. I viewed the source and found the following chunk of code implementing the "change username" widget:<code html><tr id='myopac_update_username_row' class='hide_me'>
  <td class='myopac_update_cell' colspan='3'>
    <span class='myopac_update_span'>Enter new username: </span>
    <input type='text' size='24' id='myopac_new_username' onkeydown='if(userPressedEnter(event)) myOPACUpdateUsername();'></input>
    <span class='myopac_update_span'>
      <button onclick='myOPACUpdateUsername();'>Submit</button>
    </span>
    <span class='myopac_update_span'>
      <button onclick='hideMe($(&quot;myopac_update_username_row&quot;));'>Cancel</button>
    </span>
  </td>
</tr></code> From this, I was able to determine that the JavaScript method of interest was ''myOPACUpdateUsername()''.
  - Then I grepped for the function definition of that method. There are two main directories containing JavaScript files of interest to the OPAC:<code bash>dan@denials:~/source/Evergreen-trunk$ grep "function\s*myOPACUpdateUsername" Open-ILS/web/opac/common/js/*js
dan@denials:~/source/Evergreen-trunk$ grep "function\s*myOPACUpdateUsername" Open-ILS/web/opac/skin/default/js/*js
Open-ILS/web/opac/skin/default/js/myopac.js:function myOPACUpdateUsername() {
</code> Okay -- success, it's in ''Open-ILS/web/opac/skin/default/js/myopac.js''!
  - The pertinent chunk of code in the function definition is as follows: <code javascript>function myOPACUpdateUsername() {
	var username = $('myopac_new_username').value;

...

	/* first see if the requested username is taken */
	var req = new Request(CHECK_USERNAME, G.user.session, username);
	req.send(true);
	var res = req.result();
	if( res && res == G.user.id() ) {
		alertId('myopac_username_dup');
		return;
	}</code> Calls to OpenSRF methods from JavaScript are routed through a ''Request'' object. The ''Request'' object is defined in ''Open-ILS/web/opac/common/js/opac_utils.js''. 
  - The first parameter in the ''Request'' object initializer is a variable defined in ''Open-ILS/web/common/js/config.js''.<code javascript>
var CHECK_USERNAME = 'open-ils.actor:open-ils.actor.username.exists';
</code> This is the OpenSRF method of interest. So now we just have to find it and change its behaviour.
  - So we're back to grepping the source code. At this point, most of the OpenSRF methods are defined in Perl, so let's start there:<code bash>
dan@denials: $ find Open-ILS/src/perlmods/OpenILS/ -name "*.pm" -exec grep -H open-ils.actor.username.exists {} \;
Open-ILS/src/perlmods/OpenILS/Application/Actor.pm:     api_name        => 'open-ils.actor.username.exists'
</code> Bingo!
  - The relevant Perl code that implements that method is:<code perl>
__PACKAGE__->register_method(
	method => 'usrname_exists',
	api_name	=> 'open-ils.actor.username.exists',
	signature => q/
		Returns 1 if the requested username exists, returns 0 otherwise
	/
);

sub usrname_exists {
	my( $self, $conn, $auth, $usrname ) = @_;
	my $e = new_editor(authtoken=>$auth);
	return $e->event unless $e->checkauth;
	my $a = $e->search_actor_user({usrname => $usrname, deleted=>'f'}, {idlist=>1});
	return $$a[0] if $a and @$a;
	return 0;
}</code> To make this method return ''null'' if the username does not exist, we change the final ''return 0;'' to ''return undef;'' to return an empty payload from the method. We also adjust the method documentation accordingly.
  - We adjust the code in ''myopac.js'' to handle the new expectations and sprinkle some comments while we're there:<code javascript>
function myOPACUpdateUsername() {
	var username = $('myopac_new_username').value;
...
	/* first see if the requested username is taken */
	var req = new Request(CHECK_USERNAME, G.user.session, username);
	req.send(true);
	var res = req.result();
	/* If the username does not already exist, res will be null;
	 * we can move on to updating the username.
	 * 
	 * If the username does exist, then res will be the user ID.
	 * G.user.id() gives us the currently authenticated user ID.
	 * If res == G.user.id(), we try to update the username anyways.
	 */
	if( res != null && res != G.user.id() ) {
		alertId('myopac_username_dup');
		return;
	}</code> Then we test it out -- and find that it works successfully. Hurray!
  - Now let's find any other places that method is called and repair them accordingly. First we'll check for the OpenSRF method name:<code bash>
dan@denials: $ find . -exec grep -H "open-ils.actor.username.exists" {} \; | grep -v svn
./Open-ILS/src/perlmods/OpenILS/Application/Actor.pm:   api_name        => 'open-ils.actor.username.exists',
./Open-ILS/xul/staff_client/build/chrome/content/OpenILS/util/config.js:var CHECK_USERNAME                              = 'open-ils.actor:open-ils.actor.username.exists';</code> These are innocuous.
  - Now we check for the ''CHECK_USERNAME'' constant in JavaScript files:<code bash>
dan@denials: $ find . -exec grep -H "CHECK_USERNAME" {} \; | grep -v svn
./Evergreen/xul/staff_client/server/patron/ue_config.js:        var req = new Request(CHECK_USERNAME, SESSION, usrname);
./Open-ILS/xul/staff_client/server/patron/ue_config.js: var req = new Request(CHECK_USERNAME, SESSION, usrname);
./Open-ILS/xul/staff_client/build/server/patron/ue_config.js:   var req = new Request(CHECK_USERNAME, SESSION, usrname);
./Open-ILS/xul/staff_client/build/chrome/content/OpenILS/util/config.js:var CHECK_USERNAME                              = 'open-ils.actor:open-ils.actor.username.exists';
./Open-ILS/web/opac/skin/default/js/myopac.js:  var req = new Request(CHECK_USERNAME, G.user.session, username);
./Open-ILS/web/opac/common/js/config.js:var CHECK_USERNAME                              = 'open-ils.actor:open-ils.actor.username.exists';
</code> So we have a few instances of the call being made from the staff client. We'll modify these the same way as we did ''myopac.js''.